The present invention relates to a process for producing large-sized steel pipes of rectangular or square cross section for use in buildings and other structures and more particularly to the production of such pipe from a single plate.
Such a large-sized steel pipe having a rectangular or square cross section (hereinafter referred to simply as "square steel pipe") is usually produced by bending two steel plates into U-shaped cross sections respectively and joining the bent plates by butt welding. Since the steel pipe thus produced has two weld lines, the welding material and operation account for a substantial portion of the cost of the steel pipe, and removal of the strain resulting from the welding operation is similarly costly. These factors present difficulties in reducing the manufacturing cost of steel pipes. Whereas steel pipes of small size can be produced by bending a steel plate and welding the plate along a single line, production of large-sized steel pipes, 350 mm.times.350 mm or larger in cross section, usually involves many difficulties in the bending of the material and in the other processing steps, so that it has not been practical heretofore to produce large-sized steel pipes from a single plate.